The grain-oriented electrical steel sheet is a soft magnetic material, a crystal orientation of which being highly accumulated into Goss orientation ({110}<001>), and is mainly used in an iron core for transformers, an iron core for electric motors or the like. Among them, the grain-oriented electrical steel sheets used in the transformer are strongly demanded to have low iron loss for reducing no-load loss (energy loss). As a way for decreasing the iron loss, it is known that decrease of sheet thickness, increase of Si addition amount, improvement of crystal orientation, application of tension to steel sheet, smoothening of steel sheet surface, refining of secondary recrystallization structure and so on are effective.
As a technique for refining secondary recrystallized grains among the above ways are proposed a method of performing rapid heating during decarburization annealing as disclosed in Patent Documents 1-4, a method of performing rapid heating just before decarburization annealing to improve primary recrystallization texture, and so on. For instance, Patent Document 1 discloses a technique of providing a grain-oriented electrical steel sheet with a low iron loss by heating a cold rolled steel sheet rolled to a final thickness up to a temperature of not lower than 700° C. in a non-oxidizing atmosphere having PH2O/PH2 of not more than 0.2 at a heating rate of not less than 100° C./s just before decarburization annealing. Also, Patent Document 3 and the like disclose a technique wherein electrical steel sheets having excellent coating properties and magnetic properties are obtained by heating a temperature zone of not lower than 600° C. at a heating rate of not less than 95° C./s to not lower than 800° C. and properly controlling an atmosphere of this temperature zone.
In these techniques of improving the primary recrystallized texture by the rapid heating, the heating rate is unambiguously defined with respect to a temperature range of roughly from room temperature to not lower than 700° C. as a temperature range for rapid heating. According to this technical idea, it is understood that the improvement of the primary recrystallized texture is attempted by raising the temperature close to a recrystallization temperature for a short time to suppress growth of γ-fibers ({111} fiber structure), which is preferentially formed by usual heating rate, and promote generation of {110}<001> structure as nuclei for secondary recrystallization. By the application of this technique can be refined secondary recrystallized grains to improve iron loss.
In the above technique of performing the rapid heating, it is said that large effects are obtained at a heating rate of not less than about 80° C./s or a further higher heating rate though the effect by the rapid heating may be developed at not less than 50° C./s by properly controlling the rolling conditions as disclosed in Patent Document 5. In order to increase the heating rate, however, there are problems that special and large-size heating installations such as induction heating, electric heating and the like are required and input of large energy is required in a short time. Also, there is a problem that the form of the steel sheet is deteriorated to lower sheet threading performance in the production line due to sharp temperature change through the rapid heating.